There is a growing need for personal protective apparel that guards against toxic chemical and biological agents. These agents may be                (a) accidentally released in a chemical manufacturing plant, in a scientific or medical laboratory or in a hospital;        (b) released intentionally during wartime by a government to attack the military forces of the opposition; or        (c) released during peacetime by criminal or terrorist organizations with the purpose of creating mayhem, fear and widespread destruction.        
For this reason, the United States military and other defense organizations of countries all over the world have sought to provide adequate protection against chemical and biological warfare agents. The need for such protective apparel also extends to police departments, fire departments, emergency responders and health care providers. These organizations are responsible for providing assistance and relief after a catastrophic release of chemical or biological toxins, but they cannot discharge their responsibilities without adequate protection (“Chemical Protective Clothing for Law Enforcement Patrol Officers and Emergency Medical Services when Responding to Terrorism with Chemical Weapons”, Arca, V. J. and Marshall, S. M., in report of the Chemical Weapons, Improved Response Program, U.S. Army Soldier and Biological Chemical Command, November 1999).
According to the Handbook of Chemical and Biological Warfare Agents (D. Hank Ellison, CRC Press, Boca Raton, Fla., 1st edition, 1999), most chemical warfare toxins are fatal at concentrations as low as 1 part per million (ppm). Hence, to provide adequate protection from chemical warfare agents, a protective suit has to be almost impermeable to such chemicals. It is not difficult to devise structures that are impermeable to toxic chemical vapors and liquids, but such structures are also hot, heavy and uncomfortable to wear. The degree of comfort offered by a protective suit is largely determined by the amount of water vapor that can permeate through the protective fabric. The human body continuously perspires water vapor as a method for controlling body temperature. When a protective fabric hinders the loss of water vapor from the body, the transpirational cooling process is hindered, which leads to personal discomfort. When a protective suit allows little or no loss of water vapor, extreme heat stress or heat stroke can result in a short period of time. Hence, in addition to offering the highest levels of protection against toxic chemicals and liquids, a practical chemical and biological protective suit generally should have relatively high water vapor transmission rates. Desirable protective structure are also light in weight and offer the same high level of protection over a long period of time.
Some currently available protective garments offer the same constant level of protection at all times, but often, protection is only needed when a toxic chemical or biological agent is present in the environment. Further, comfort is typically sacrificed at the expense of protection or vice versa. A garment is needed that provides a variable and controllable permeability.
U.S. application Ser. No. 11/118,961, which is incorporated in its entirety as a part hereof for all purposes, addresses these problems by providing an adaptive membrane structure having two membranes and means to respond to an actuating stimulus (for example, an electrostatic force) that will move one membrane into contact with the other such that the permeability of the structure to gas, vapor, liquid and/or particulates is decreased.
An adaptive membrane structure as described in this reference may, however, be used to protect against extremely toxic agents, such as those encountered in chemical and biological warfare. If the actuating stimulus should fail for some reason to bring the adjacent surfaces of the two membranes together in a manner such that a good quality seal is provided, protection against toxic agents may not be adequate for some conditions. Malfunction of the actuating stimulus, and a poor seal between adjacent membranes as a consequence, could result from a variety of causes such as damage to the membrane structure. Under other circumstances, however, the actuating stimulus could simply be too weak to adequately perform the job of moving adjacent membranes together in a manner such that a good seal between them is obtained.
It is thus desirable in such situations to consider an alternative to the above described adaptive membrane structure wherein the seal between adjacent membranes would not be provided as a result of an actuating stimulus.